Tank mixing is the practice of combining two or more crop protection products, such as a fungicide and an insecticide, into a single application tank. This process is commonly used by growers to save time and labor by addressing multiple pest or disease issues simultaneously. While combining products is often effective, it demands strict caution and adherence to manufacturer instructions. Failing to ensure compatibility can result in financial loss, equipment damage, and ineffective pest control. The success of this dual-action application hinges on understanding the chemical and physical interactions that occur when different formulations are mixed.
Understanding Physical and Chemical Compatibility
A successful tank mix must maintain a uniform suspension in the spray tank, which involves two distinct forms of compatibility: physical and chemical. Physical incompatibility is an immediate, visible problem where the products fail to mix evenly within the water carrier. This failure can manifest as flocculation, where fine particles clump together, or as gelling and thickening of the solution.
These physical issues prevent a uniform application and can quickly clog screens, nozzles, and pump parts within the sprayer equipment. Factors like water hardness, dissolved minerals, and extreme water temperatures can influence whether products remain stably suspended or separate into distinct layers. Agitation is necessary to keep the mixture homogeneous, but if the products are physically incompatible, constant stirring cannot prevent separation or the settling out of solids.
Chemical incompatibility is a less visible but equally destructive issue, involving reactions between the active ingredients or the inert carriers. This often occurs when the mixture’s overall pH level shifts dramatically, causing a process called alkaline or acid hydrolysis. In alkaline hydrolysis, a high pH can cause certain pesticide molecules to break down rapidly. This degradation can render the active ingredient ineffective before it even leaves the sprayer nozzle.
The risk of chemical incompatibility is closely tied to the product’s formulation type, such as Wettable Powders (WPs) or Emulsifiable Concentrates (ECs). Mixing an acidic formulation with an alkaline one can trigger an adverse reaction that destroys one or both active ingredients. A solution that looks perfectly stable in the tank may still be chemically compromised, leading to poor results in the field.
Risks to Plants and Efficacy (Phytotoxicity and Synergistic Effects)
Even when a mixture is physically stable in the tank, the combination can still lead to undesirable effects once applied to the plant. The most common negative outcome is phytotoxicity, which refers to plant injury caused by the chemical treatment. Symptoms of phytotoxicity can range from minor leaf spotting and discoloration to severe leaf burn, necrosis, and stunted growth.
Mixing two products can increase the overall concentration of solvents or oils in the spray solution, which are common components in Emulsifiable Concentrate (EC) formulations. These solvents enhance the penetration of the active ingredients into the plant tissue, but they can also cause the plant’s protective wax layer to break down excessively. This enhanced penetration can lead to chemical burn, especially under hot, humid, or sunny conditions. Even if each product is safe on its own, their combination can create an unexpectedly harsh treatment for the plant.
Efficacy Loss (Antagonism)
A concern is the potential for efficacy loss, known as antagonism, where the combined effect is less than the sum of the individual products. In an antagonistic reaction, one product may neutralize or inhibit the biological activity of the other, meaning the pest or disease is not effectively controlled. This outcome is costly, requiring a second, separate application to achieve the desired management.
Synergistic Effects
Tank mixing can also result in synergistic effects, where the combined toxicity is greater than expected. While synergy is sometimes beneficial for pest control, an excessive reaction can lead to severely increased phytotoxicity on the target plant. Synergistic mixtures have also been shown to increase toxicity to non-target organisms, such as beneficial insects and pollinators.
Practical Guide to Successful Tank Mixing
The first and most important step before combining any products is to thoroughly review the product labels for both the fungicide and the insecticide. The label often contains specific instructions regarding compatibility, including warnings against mixing with certain other products or explicit instructions for a required mixing sequence. If the label prohibits tank mixing, that instruction must be followed without exception. If no specific mixing instructions are present, a small-scale compatibility test, known as a jar test, should be performed before mixing a full tank. This test simulates the conditions in the spray tank using the same water source and the exact proportional amounts of each product.
Start by filling a clear glass jar about halfway with the carrier water, then add each product in the correct sequence, stirring thoroughly after each addition. The proper mixing sequence is crucial for preventing physical incompatibility and generally follows the order of dry formulations first, followed by liquid formulations. The typical sequence is:
- Water Soluble Bags (WSB)
- Wettable Powders (WP)
- Dry Flowables (DF)
- Liquid Flowables (LF) or Suspension Concentrates (SC)
- Emulsifiable Concentrates (EC)
- Water-soluble solutions (SL)
Any adjuvants or surfactants should be added last. After mixing all components, the jar should be capped, shaken vigorously for about 30 seconds, and then allowed to stand undisturbed for 15 to 30 minutes.
During this observation period, check the mixture for any signs of physical incompatibility such as clumping, gelling, excessive foam, or the formation of a sludge layer at the bottom. If the mixture remains uniform and pourable, it is physically compatible. The jar test does not guarantee chemical compatibility or the absence of phytotoxicity, which can only be confirmed by a small-scale application test. If any negative physical signs are observed, a full-scale tank mix should not be attempted.